JP2010283255A - Nitride semiconductor solar cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To achieve a tandem structure good for a solar cell comprising a GaN-based semiconductor. <P>SOLUTION: A nitride semiconductor solar cell includes: a first semiconductor layer 102 comprising a III-V nitride semiconductor and including a first pn junction; and a second semiconductor layer 104 comprising a III-V nitride semiconductor and including a second pn junction different in bandgap from the first semiconductor layer 102. A first contact layer 103, which is formed in ohmic contact with the first semiconductor layer 102 and the second semiconductor layer 104 and is an oxide layer including zinc, is formed between the first semiconductor layer 102 and the second semiconductor layer 104. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a thin-film nitride semiconductor solar cell that can be used for photovoltaic power generation.

  Energy saving is attracting attention for the prevention of global warming, and so-called photovoltaic power generation, in which electric power is obtained by sunlight, is attracting attention among technologies for realizing this. A device for realizing this is a solar cell, and electric power can be obtained by taking out an electron-hole pair generated when a semiconductor pn junction is irradiated with light as a photocurrent.

In order to increase the efficiency of the solar cell, it is necessary to use sunlight including a wide wavelength band as efficiently as possible, and there is a limit to improvement in a pn junction having a single forbidden bandwidth. In order to solve this problem, a tandem (stack) structure in which a plurality of pn junctions having different forbidden bandwidths are stacked and the stacked pn junctions are connected by p ⁺ n ⁺ junctions is proposed. The p ⁺ n ⁺ junction functions as an ohmic electrode with respect to adjacent pn junctions by so-called tunnel current. With this configuration, each pn junction can efficiently absorb sunlight and perform photoelectric conversion, so that a more efficient operation is possible. The solar cells that have been used so far have been made of polycrystalline silicon or amorphous silicon-based materials, but in order to convert the shorter wavelength region to high efficiency, it is laminated using a material with a larger forbidden bandwidth. There is a need to.

Group III-V nitride compound semiconductors represented by gallium nitride (GaN) (general formula In _x Al _y Ga _1-xy N (where 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ x + y ≦ (Hereinafter referred to as GaN-based semiconductor) is a material that can cover a wide wavelength band from the ultraviolet region to the infrared region by changing its composition. Therefore, if a GaN-based semiconductor can be stacked as a tandem structure, a more efficient solar cell can be expected.

Japanese Examined Patent Publication No. 63-048197 JP 2008-235878 A

However, since the GaN-based semiconductor cannot sufficiently increase the carrier concentration value of the p-type layer in particular, it is difficult to realize a p ⁺ n ⁺ junction capable of obtaining a necessary tunnel current with the GaN-based semiconductor. That is, a solar cell using a GaN-based semiconductor has a problem that a good tandem structure cannot be realized and high-efficiency operation cannot be performed.

  In view of the above-described conventional problems, an object of the present invention is to realize a good tandem structure in a solar cell made of a GaN-based semiconductor.

  In order to achieve the above object, according to the present invention, a nitride semiconductor solar cell is provided with an oxide layer containing zinc between a plurality of nitride semiconductor layers including a pn junction.

  Specifically, a nitride semiconductor solar cell according to the present invention includes a first semiconductor layer made of a III-V nitride semiconductor and including a first pn junction, and a first semiconductor layer made of a III-V nitride semiconductor. The semiconductor layer is formed in ohmic contact with each of a second semiconductor layer including a second pn junction having a different forbidden band width, and between the first semiconductor layer and the second semiconductor layer, and zinc is used. And an oxide layer including the oxide layer.

  According to the nitride semiconductor solar cell of the present invention, each of the group III-V nitride semiconductors is formed in ohmic contact with each of the first semiconductor layer and the second semiconductor layer including the pn junction, In addition, since the oxide layer including zinc is provided, good ohmic contact can be realized between the first semiconductor layer and the second semiconductor layer. Thereby, since a favorable tandem structure can be realized, high efficiency of the nitride semiconductor solar cell can be realized.

  In the nitride semiconductor solar battery of the present invention, the oxide layer containing zinc may contain at least one of magnesium and cadmium.

  In this manner, the forbidden band width of the oxide layer containing zinc can be designed to an appropriate value, and light absorption by the oxide layer can be suppressed, so that a solar cell with higher conversion efficiency can be obtained.

  In the nitride semiconductor solar cell of the present invention, incident light is incident from the second semiconductor layer side, the forbidden band width of the second pn junction is larger than the forbidden band width of the first pn junction, and the oxide layer The forbidden width is preferably larger than the forbidden band width of the first pn junction.

  In this manner, the value of each forbidden bandwidth is designed to be large on the incident side between the first pn junction and the second pn junction and between the oxide layer containing zinc and the second pn junction. As a result, light absorption at the second pn junction is increased, while light absorption in the oxide layer is suppressed, so that a thin-film solar cell with higher conversion efficiency can be realized.

  In the nitride semiconductor solar cell of the present invention, periodic uneven shapes may be formed in the first semiconductor layer, the oxide layer, and the second semiconductor layer.

  Thus, the crystallinity of the first semiconductor layer and the like can be improved and light absorption can be increased, so that a more efficient thin film solar cell can be obtained.

  The nitride semiconductor solar cell of the present invention may further include a substrate that holds the first semiconductor layer, the oxide layer, and the second semiconductor layer.

  In this way, it is possible to realize a thin-film solar cell that can further increase light absorption.

  In this case, graphite can be used for the substrate. By doing so, it is possible to realize a solar cell with lower cost and higher efficiency than when sapphire or the like is used for the growth substrate of each semiconductor layer.

  In this case, the substrate may have a periodic uneven shape on its main surface. Thus, the crystallinity of the first semiconductor layer and the like can be improved and light absorption can be increased, so that a more efficient thin film solar cell can be obtained.

  The nitride semiconductor solar cell of the present invention may further include an aluminum nitride layer formed so as to be in contact with the substrate between the substrate and the first semiconductor layer.

  Thus, the crystallinity of the first semiconductor layer and the like can be further improved.

  The nitride semiconductor solar cell of the present invention includes a first semiconductor layer and a second semiconductor between at least one of the first semiconductor layer and the oxide layer and between the second semiconductor layer and the oxide layer. A third semiconductor layer made of a group III-V nitride semiconductor formed at a lower temperature than the layer may be further provided.

  Thus, by providing the third semiconductor layer, diffusion of zinc from the oxide layer containing zinc to the first semiconductor layer and the second semiconductor layer is suppressed. Since the ohmic contact with the second semiconductor layer becomes better, a more efficient thin-film solar cell can be obtained.

According to the nitride semiconductor solar cell according to the present invention, a good tandem structure can be realized,
A more efficient solar cell can be obtained.

It is sectional drawing which shows the nitride semiconductor solar cell which concerns on the 1st Embodiment of this invention. It is a graph which shows the X-ray-diffraction rocking curve in the GaN growth on the graphite sheet which concerns on the 1st Embodiment of this invention, and compared the case where AlN is used for the initial layer, and the case where GaN is used. It is sectional drawing which shows the nitride semiconductor solar cell which concerns on the 2nd Embodiment of this invention. (A) And (b) shows the uneven | corrugated shape formed in the surface of the graphite sheet based on the 2nd Embodiment of this invention, (a) is a top view, (b) is IVb-IVb of (a). It is sectional drawing in a line. It is sectional drawing which shows the nitride semiconductor solar cell which concerns on the modification of the 2nd Embodiment of this invention. (A)-(d) is sectional drawing of the order of a process which shows the manufacturing method of the nitride semiconductor solar cell which concerns on the modification of the 2nd Embodiment of this invention.

(First embodiment)
A first embodiment of the present invention will be described with reference to FIGS.

As shown in FIG. 1, the nitride semiconductor solar cell according to the first embodiment has an initial layer 101 made of, for example, aluminum nitride (AlN) having a thickness of about 20 nm on a graphite sheet 100 that is a flexible substrate. , First semiconductor layer 102 including a pn junction made of indium gallium nitride (In _0.4 Ga _0.6 N), second contact layer 105 made of zinc oxide (ZnO), aluminum gallium nitride (Al _0.2) A third semiconductor layer 106 including a pn junction made of Ga _0.8 N) and a third contact layer 107 made of zinc oxide (ZnO) are sequentially formed by epitaxial growth.

  A surface electrode 108 made of, for example, indium tin oxide (ITO) is formed on the third contact layer 109, and further, the third contact layer 109 covers the entire surface including the surface electrode 108, For example, an antireflection film 109 made of silicon nitride (SiN) is formed. A back electrode 110 made of gold (Au) is formed on the surface of the graphite sheet 100 opposite to the initial layer 101.

  Here, the third semiconductor layer 106 that receives the incident light mainly absorbs the incident light in the ultraviolet region and converts it into a photocurrent, and the second semiconductor layer 104 thereunder mainly receives the incident light in the blue-violet region. It absorbs and converts it into a photovoltaic current, and the first semiconductor layer 102 below it absorbs incident light mainly in the visible region and converts it into a photovoltaic current.

  In the first embodiment, before forming the first semiconductor layer 102, the initial layer 101 made of AlN is formed on the main surface of the graphite sheet 100, for example, by metal organic chemical vapor deposition (Metal Organic Chemical Vapor Deposition). : MOCVD) method or pulsed laser deposition (PLD) method. AlN has an effect of improving the crystallinity of the GaN-based semiconductor layer on the graphite sheet 100.

  FIG. 2 shows a rocking curve by X-ray diffraction when a GaN layer is formed on each initial layer 101 when AlN is used as the initial layer 101 on the graphite sheet 100 and when GaN is used. The result of having measured GaN (0002) peak is shown. Both layers use MOCVD for crystal growth. As can be seen from FIG. 2, the crystallinity of the GaN layer formed on the initial layer made of AlN is improved by using AlN as the initial layer.

To manufacture the nitride semiconductor solar cell shown in FIG. 1, a first semiconductor layer made of In _0.4 Ga _0.6 N and including a pn junction is formed on the initial layer 101 made of AlN by MOCVD. 102 is formed. Subsequently, a low-resistance first contact layer 103 made of ZnO is formed by a PLD method. Thereafter, similarly, the second semiconductor layer 104 made of GaN and including a pn junction, the second contact layer 105 made of ZnO, and Al _0.2 Ga _0.8 N are formed on the first contact layer 103. A third semiconductor layer 106 including a pn junction and a third contact layer 107 made of ZnO are sequentially formed. For example, magnesium (Mg) can be used as the p-type dopant for forming the p-type layer in the pn junction, and silicon (Si) is used as the n-type dopant for forming the n-type layer. Can do.

  Here, the thickness of the first contact layer 103 and the second contact layer 105 needs to have sufficient transparency with respect to the wavelength absorbed by the pn junction located on the lower side. Therefore, the thickness of the contact layers 103 and 105 is preferably about 10 nm or less. In addition, the thickness of the third contact layer 107 formed on the top may be about 100 nm.

  The thicknesses of the first semiconductor layer 102, the second semiconductor layer 104, and the third semiconductor layer 106 may be about 0.5 μm, 0.3 μm, and 0.3 μm, respectively. Furthermore, each of the semiconductor layers 102, 104, and 106 is desirably a so-called pin-type junction in which an undoped layer is formed between the pn junctions. By increasing the amount of light absorption by the undoped layer in the pin-type junction, the conversion efficiency of the solar cell can be improved. Note that the thickness of each undoped layer that absorbs light at the pn junction constituting each of the semiconductor layers 102, 104, and 106 is preferably sufficiently thick so as not to increase the series resistance, for example, about 500 nm. .

Furthermore, in the first embodiment, the contact layers 103, 105, and 107 made of ZnO have a larger forbidden band width than the semiconductor layers 102, 104, and 106 formed on the lower side, and thus absorb light. It is desirable to function as a transparent conductive film that is suppressed. Specifically, magnesium (Mg), cadmium (Cd), or both of which can adjust the forbidden band width of ZnO may be added to ZnO constituting each contact layer 103, 105, 107. For example, the first contact layer 103 is desirably made of Zn _0.8 Cd _0.2 O having a larger forbidden band width than In _0.4 Ga _0.6 N of the first semiconductor layer 102. The second contact layer 105 is _{preferably made of} Zn _0.9 Mg _0.1 O having a larger forbidden band width than GaN, and the third contact layer 107 is made of Al _0.2 Ga _0.8 N. Zn _0.5 Mg _0.5 O having a larger forbidden band than that is desirable.

  In addition, the initial layer 101 and the semiconductor layers 102, 104, and 106 are provided above and below the first contact layer 103, above and below the second contact layer 105, and below the third contact layer 107. There may be provided a diffusion suppression layer made of InAlGaN grown at a temperature lower than the growth temperature, and suppressing the diffusion of zinc (Zn) into each of the semiconductor layers 102, 104, and 106. For example, the diffusion suppression layer provided on the upper and lower sides of the first contact layer 103 may have a thickness of 5 nm and a composition of AlN.

  In the first embodiment, the MOCVD method is used for the growth method of each semiconductor layer 102, 104, 106, and the PLD method is used for the growth method of each contact layer 103, 105, 107. Any one of them may be used. When the growth method is limited to one type, a semiconductor layer or the like in the middle of growth is continuously grown without being exposed to the atmosphere, so that a natural oxide film or the like may be formed at the interface of the semiconductor layer or the like in the middle of the growth. Absent. Therefore, the nitride semiconductor solar cell according to the first embodiment can be formed without increasing the value of the series resistance.

  As described above, in the first embodiment, the first contact layer 103 made of a conductive oxide containing zinc (Zn) is replaced with the first semiconductor layer 102 made of a nitride semiconductor containing a pn junction. A second contact layer 105 is formed between the second semiconductor layer 104 and the second semiconductor layer 104, and a second contact layer 105 is formed between the second semiconductor layer 104 and the third semiconductor layer 106 each including a pn junction. Thereby, it becomes possible to form a tandem solar cell having good ohmic contact, and a more efficient nitride semiconductor solar cell can be realized.

(Second Embodiment)
Hereinafter, a second embodiment of the present invention will be described with reference to FIGS. In FIG. 3, the same components as those in FIG.

As shown in FIG. 3, the graphite sheet 100 </ b> A according to the second embodiment has a periodic uneven shape formed on its main surface. As described above, when the concavo-convex shape is provided on the main surface of the growth substrate on which the GaN-based semiconductor layer is grown, the initial layer 101 made of AlN and the first In _0.4 Ga _0.6 N made thereon are formed. Periodicity is generated in the formation of initial growth nuclei such as the semiconductor layer 102 of the semiconductor layer 102. This periodicity suppresses in-plane rotation of the crystal structure, and as a result, the crystallinity of each semiconductor layer 102, 104, etc. is further improved. For this reason, the highly efficient operation | movement of a solar cell is attained.

  In addition, by forming a concavo-convex shape on the main surface of the graphite sheet 100A, the first semiconductor layer 102 and the like including the pn junction includes so-called nonpolar surfaces including many surfaces other than the (0001) plane in the crystal plane. Become. For this reason, each of the semiconductor layers 102, 104, and 106 is not affected by the intrinsic polarization generated in the GaN-based semiconductor, so that the solar cell can operate with high efficiency. Further, since this uneven shape is also formed on the surface of the third contact layer 107 made of ZnO, the amount of incident light absorbed is increased, so that a more efficient solar cell can be realized.

  FIG. 4A and FIG. 4B show an example of a periodic uneven shape formed on the main surface of the graphite sheet 100A. Here, as shown in FIGS. 4A and 4B, a plurality of planar hexagonal recesses 100a are arranged to form hexagonal vertices. The length of one side of each recess 100a may be, for example, about 0.5 μm, and the interval between the placement of the recesses 100a may be about 20 μm. By adopting such a shape, as described above, when the (0001) plane of GaN is formed, each recess 100a functions as a nucleus for crystal formation. GaN having excellent crystallinity can be formed.

  In order to form the periodic recess 100a in the graphite sheet 100A, a resist pattern having the periodic recess 100a opened is formed on the main surface of the graphite sheet 100A by, for example, lithography, and the formed resist pattern is masked. Then, etching, for example, ashing in an oxidizing atmosphere may be performed.

(One Modification of Second Embodiment)
Hereinafter, a modification of the second embodiment of the present invention will be described with reference to FIG. In FIG. 5, the same components as those in FIG.

  As shown in FIG. 5, in the nitride semiconductor solar cell according to this modification, the graphite sheet 100 </ b> A and the initial layer 101 with the irregular shape formed on the main surface are removed, and the back electrode 110 </ b> A is the first semiconductor layer 102. The structure formed on the surface opposite to the first contact layer 102 in FIG. Note that gold (Au) can be used for the back electrode 110A.

  Hereinafter, the manufacturing method of the nitride semiconductor solar cell according to this modification configured as described above will be described with reference to FIG.

  First, as shown to Fig.6 (a), periodic uneven | corrugated shape is formed in the main surface of the graphite sheet 101a with the method mentioned above.

Next, as shown in FIG. 6B, as in the first embodiment, the initial layer 101 made of AlN, the first semiconductor layer 102 made of In _0.4 Ga _0.6 N, and ZnO A first contact layer 103 made of GaN, a second semiconductor layer 104 made of GaN, a second contact layer 105 made of ZnO, a third semiconductor layer 102 made of Al _0.2 Ga _0.8 N, and a second semiconductor layer made of ZnO. Three contact layers 107 are sequentially formed.

  Next, as shown in FIG. 6C, the graphite sheet 101A is peeled from the formed GaN-based semiconductor layer. Since the graphite sheet 101A is flexible, it can be easily peeled off using an adhesive sheet or the like. Subsequently, the initial layer 101 exposed by peeling off the graphite sheet 101A is removed by dry etching using, for example, chlorine gas. Thereafter, a back electrode 110A made of Au is formed on the back surface of the first semiconductor layer 102 exposed by removing the initial layer 101 by vacuum vapor deposition or plating.

  Next, as shown in FIG. 6D, a surface electrode 108 and an antireflection film 109 are formed on the third contact layer 107, thereby obtaining the nitride semiconductor solar cell shown in FIG.

  In the first embodiment, the second embodiment, and the modifications thereof, the number of stacked pn junctions (tandem number) is three, but the forbidden band width is gradually reduced from the upper layer toward the substrate side. Four or more semiconductor layers formed as described above may be stacked.

  In each embodiment and its modification, the MOCVD method or the PLD method is used for the growth of the GaN-based semiconductor. However, the present invention is not limited to this, and other crystal growth methods such as a molecular beam epitaxy (MBE) method are used. May be. Further, the semiconductor layers and the contact layers may be continuously grown so that the interface between the layers does not deteriorate.

  Further, although a graphite sheet is used as a growth substrate for a GaN-based semiconductor, the growth substrate is not limited to a graphite sheet, and for example, inexpensive silicon (Si) may be used.

As described above, the nitride semiconductor solar cell according to each embodiment includes a semiconductor layer 102 including a pn junction made of nitride semiconductors having different forbidden band widths on a low-cost and flexible graphite sheet 100. 104 and 106 are stacked, and contact layers 103, 105, and 107 made of, for example, ZnO are formed between the semiconductor layers. This pn junction is composed of, for example, In _0.4 Ga _0.6 N, GaN, and Al _0.2 Ga _0.8 N from the graphite sheet 100 side, and absorbs sunlight in the visible region, the blue-violet region, and the ultraviolet region, respectively. Thus, an electron hole pair is formed. Since the contact layer made of ZnO formed between the nitride semiconductor layers realizes a good ohmic contact with the semiconductor layers including pn junctions formed above and below and transmits incident light, the nitride semiconductor solar cell Enables high-efficiency operation.

  The nitride semiconductor solar cell according to the present invention can realize a good tandem structure and is useful for a thin-film nitride semiconductor solar cell that can be used for solar power generation.

100 Graphite sheet (flexible substrate)
100A graphite sheet (flexible substrate)
100a dent 101 initial layer 102 first semiconductor layer 103 first contact layer 104 second semiconductor layer 105 second contact layer 106 third semiconductor layer 107 third contact layer 108 surface electrode 109 antireflection film 110 back surface Electrode 110A Back electrode

Claims (9)

A first semiconductor layer made of a III-V nitride semiconductor and including a first pn junction;
A second semiconductor layer comprising a group III-V nitride semiconductor and including a second pn junction having a forbidden band width different from that of the first semiconductor layer;
A nitride semiconductor solar cell comprising an oxide layer formed in ohmic contact with each of the first semiconductor layer and the second semiconductor layer and containing zinc.   The nitride semiconductor solar cell according to claim 1, wherein the oxide layer includes at least one of magnesium and cadmium. Incident light is incident from the second semiconductor layer side,
The forbidden band width of the second pn junction is larger than the forbidden band width of the first pn junction;
3. The nitride semiconductor solar cell according to claim 1, wherein a forbidden width of the oxide layer is larger than a forbidden band width of the first pn junction.   4. The nitride semiconductor according to claim 1, wherein the first semiconductor layer, the oxide layer, and the second semiconductor layer have a periodic uneven shape. 5. Solar cell.   The nitride semiconductor solar cell according to any one of claims 1 to 3, further comprising a substrate that holds the first semiconductor layer, the oxide layer, and the second semiconductor layer.   The nitride semiconductor solar cell according to claim 5, wherein the substrate is made of graphite.   The nitride semiconductor solar cell according to claim 5, wherein the substrate has a periodic uneven shape formed on a main surface thereof.   The aluminum nitride layer formed so that it may contact between the said board | substrate and the said 1st semiconductor layer, and the said board | substrate is further provided, The Claim 1 characterized by the above-mentioned. Nitride semiconductor solar cells.   A temperature lower than the first semiconductor layer and the second semiconductor layer between at least one of the first semiconductor layer and the oxide layer and between the second semiconductor layer and the oxide layer. The nitride semiconductor solar cell according to claim 1, further comprising a third semiconductor layer made of a group III-V nitride semiconductor formed of

Images